Alkynes complex formation

Initial step is the formation of a dicobalthexacarbonyl-alkyne complex 5 by reaction of alkyne 1 with dicobaltoctacarbonyl 4 with concomitant loss of two molecules of CO. Complex 5 has been shown to be an intermediate by independent synthesis. It is likely that complex 5 coordinates to the alkene 2. Insertion of carbon monoxide then leads to formation of a cyclopentenone complex 6, which decomposes into dicobalthexacarbonyl and cyclopentenone 3 ... 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

Experiments with terminal acetylenes, isolation of an intermediate acetal, alkyne hydratation studies, and ab initio calculations provide substantiation of a unified mechanism that rationalizes the reactions in which the complex formation between the alkyne and the iron(III) halides is the activating step (Scheme 12) [27]. 

The strong o-donor property of NHC ligands enhances the catalytic activity in [3+2] cycloaddition by promoting the activation of internal alkynes (i.e. 26), which proceeds by the formation of a ti-alkyne complex 25 (Scheme 5.7). 

In some instances formation of ( 0) may be avoided by employing alkyne displacement reactions. Thus (32) is formed in essentially quantitative yield by reacting L(n-CsH5)(0C)2W = CRl with the bridged alkyne complex fNi2(y-Me3SiC2SiMe3)(n-C5H5)2] (9). 

Scheme 51 Formation of niobium-alkyne and tantalum-alkyne complexes.

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

Unsubstituted H2C=CH2 Upon reaction with complexes 2a or 6, ethene gives zirconacy-clopentanes such as 13 and 14 [16]. The formation of these compounds proceeds at low temperature via zirconacydopentenes 15, which disproportionate at room temperature to the starting alkyne complex and complex 14. The latter is formed quantitatively in the reaction of 15 with ethene. 

No defined complexes could be isolated from reactions of complex 1 with acetone Me2C=0. Complexes 2a and 2b react with acetone to give the zirconafuranone 2c, which is an interesting zirconocene precursor in view of its extremely good solubility in hydrocarbon solvents and because of its ability to dissociate into the alkyne complex [2f], It is also possible to cleanly substitute the bis(trimethylsilyl)acetylene unit so as to obtain the complex 47, or, alternatively, to substitute the acetone with formation of the zirconafuranone 95 (Fig. 10.14) [2f],... 

Diazabutadienes RN=CHCH=NR react with complexes 1 and 2 with liberation of the alkyne and formation of the corresponding diazadiene complexes 77 [43]. 

The reactive (TPP)Rh in Scheme II is electrogenerated from l(TPP)Rh(L)d ci in the presence of an alkene or an alkyne. The formation of an intermediate is observed. This intermediate is not detected by chemical reaction methods and was tentatively assigned as a it complex(15). Similar it complexes have been reported for ruthenium porphyrin species(17 18). 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Transition-metal-promoted cycloaddition is of much interest as a powerful tool for synthesis of carbocyclic stmcture in a single step. Utilization of carbon monoxide as a component of the cycloaddition reaction is now widely known as the Pauson-Khand reaction, which results in cyclopentenone formation starting from an alkyne, an alkene, and carbon monoxide mediated by cobalt catalyst. Although mechanistic understanding is limited, a commonly accepted mechanism is shown in Scheme 4.16. Formation of dicobalt-alkyne complex followed by alkene... 

R = H, Ph) have been obtained from the anionic [Ru(N4Meg)] and ethyne or HC CPh, respectively [51], In this case, formation of an intermediate r -alkyne complex is unlikely, the probable mechanism being deprotonation of the alkyne and coordination of the alkynyl anion followed by proton transfer. 

A sub-set of these reactions is provided by the redox rearrangements of several complexes whicdi have been extensively studied by Cormelly and coworkers [140]. Oxidation of the rj -alkyne complexes M(r] -Me3SiC2SiMe3)(CO)2(ri-arene) (M = Cr, Mo) results in formation of the vinylidene cations [M =C=C(SiMe3)2 (CO)2(ri-arene)]. ... 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

The alkynes are bonded in essentially the same way as, but less firmly than, the olefins (see Section III,R). In the hex-3-yne series, substitution of an a-hydrogen atom by a methyl group reduces the argentation constant (a measure of the silver-alkyne bond strength) by a factor of roughly 1 /3 this influence of methyl substitution on complex formation is opposite to that observed in the platinum(II) complexes (see Section IV,J). 

The chemistry of alkyne complexes is somewhat more complicated than that of alkene complexes because of the greater possibilities for -n bonding by alkynes and the tendency of some of die complexes to act as intermediates in the formation of other organometallic compounds. 

Mercury(II) oxide and acetic acid effect the cyclization of l,4-diaryloxybut-2-ynes to 4-aryloxymethylchromenes. The transformation was attributed to cyclization of the butanone which resulted from hydration of the alkyne (72JHC489). However, it has since been shown that similar butanones do not cyclize to chromenes under the cyclization conditions (78JOC3856). Instead, a mechanism is proposed which involves a charge-induced Claisen rearrangement which is triggered by 7r-complex formation between the metal ion... 

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